The present invention relates to patient-support devices and particularly to a patient detection system for detecting the presence of a patient on a patient-carrying surface of the patient-support device. More particularly, the present invention relates to a device for detecting the dielectric constant within a detection space adjacent to the patient-carrying surface, the dielectric constant corresponding to the presence or absence of the patient in the detection space, the device providing a signal in response to the dielectric constant within the detection space, the signal indicating the presence or absence of the patient in the detection space thereby indicating the presence or absence of the patient on the patient-carrying surface.
It is sometimes desirable to monitor the whereabouts of a patient, particularly when a doctor or other care giver has ordered that the patient remain on a patient-support device such as a hospital bed, a stretcher, or other device for carrying the patient. Several devices have been developed for detecting a patient exit from a patient-support device. For example, U.S. Pat. No. 5,276,432 to Travis discloses a bed exit detection mechanism for a hospital bed that relies on signals from load cells coupling the mattress to a frame of the bed. This device uses the weight measured by each load cell to determine whether the center of gravity of the patient is over a predetermined region of the patient-carrying surface of the hospital bed. In addition, U.S. Pat. Nos. 4,934,468 and 4,953,244, each of which are assigned to the assignee of the present invention, disclose a hospital bed having a support frame and a weigh frame mounted on the support frame by load cells.
It is also known to monitor the position of a person by attaching a portion of a transmitter/receiver system to the person being monitored so that when the person and thus the portion of the system attached to the person leaves a designated area, the remaining portion of the system detects the departure of the portion and thus the departure of the person and provides an indication of the person's departure. For example, U.S. Pat. No. 5,519,380 to Edwards discloses a bracelet module that is worn by the monitored person so that when the person and the bracelet module move outside of the monitored volume, an indication of departure is provided.
It is also known to monitor the position of a patient on a bed using bladders or other fluid-carrying devices positioned to lie between the patient and the bed and in fluid communication with a pressure sensor so that the pressure sensor registers a bladder pressure in response to the patient's weight, the bladder pressure indicating the presence or absence of the patient on the bladder. For example, U.S. Pat. Nos. 5,140,309 and 5,184,122, both to Gusakov, each disclose an apparatus including resilient means in the form of a tube, cell or other form of fluid passage adapted to allow fluid such as air to flow from an inlet through the outlet when a passage is not collapsed by the weight of a patient. Indicating means indicate changes in pressure in the fluid supplied by the fluid supplying means, such as when the weight of a patient collapses the passage through the resilient means. By providing resilient means having a plurality of tubes, cells or other fluid passages and separate indicating means for each resilient means, the position of the patient relative to each resilient means can be monitored.
There are also several known systems that include sensors positioned to lie beneath the patient and that provide electrical signals in response to the weight detected by the sensor so that an output signal indicating a significant change of weight acting against the sensor indicates movement of the patient to a position away from the sensor or to a position on top of the sensor. For example, U.S. Pat. Nos. 4,179,692 to Vance; 5,253,656 to Rinco et al.; 5,353,012 to Barham et al.; and 4,638,307 to Swartout disclose systems each of which include a sensor that provides an output signal in response to a change in the weight acting against the sensor.
Capacitive sensors that provide an output signal in response to the change in weight acting against the sensor are also well known. For example, U.S. Pat. Nos. 5,410,297 to Joseph et al.; 5,235,319 to Hill et al.; and 3,991,746 to Hannah each disclose patient monitoring systems employing capacitive sensors that provide a signal in response to the weight acting against the sensor.
Finally, it is also known to provide capacitive motion sensors for monitoring the movements of a person and even for measuring respiration, heartbeat, and body position of the person. For example, U.S. Pat. Nos. 4,320,766 to Alihanka et al. and 5,448,996 to Bellin et al. each disclose patient monitors including capacitive sensors. The device disclosed by the Alihanka patent can indicate that movement has occurred but cannot indicate what movement occurred or the position of the body when no movement is occurring, and the Bellin patent likewise can indicate movement but not position of the patient or that a patient has exited the bed. Monitoring the position of a patient on the support surface and determining that vacation of the bed by the patient is imminent, is shown in U.S. Pat. No. 4,633,237. An array of pressure sensitive capacitive on-off switches are used.
Recent improvements in bed mattress designs have significantly reduced peak interface pressure between the patient and the mattress by maximizing the area over which the weight of the patient is distributed rather than concentrating the interface pressure at points along the patient, for example, the patient's head, shoulder blades and heels. In addition, recent patient population data indicates that patient-s in general weigh less than in the past. Patient detection systems that rely on the weight of the patient to allow the sensor to detect the patient are rendered less effective as mattresses become more efficient at distributing the weight of the patient across the surface of the mattress and as the weight of the patient decreases. In addition, while it is desirable to minimize the interface pressure of high interface pressure points between the patient and the mattress by distributing the weight of the patient across the mattress, for example, by using mattresses including air bladder supports, interposing a sensor between the top surface of the mattress and the patient significantly reduces the effectiveness of the mattress at distributing the weight of the patient. In addition, for applications in which it is advantageous to keep the patient dry and maintain the temperature of the patient at a desired temperature through the use of a "low air loss" mattress that allows a very small amount of air to escape the mattress and blow on the patient, interposing a sensor between the patient and the mattress reduces the effectiveness of the low air loss feature.
What is needed is a sensor that can sense the position of a patient relative to a patient-carrying surface of a patient-support device that can be positioned to lie away from the patient allowing the sensor to be placed away from the patient-carrying surface. In addition, the sensor should not require the attachment of a portion of the system to the patient. In addition, the sensor should detect the presence of a patient within a window of detection that can be shaped through the use of multiple sensing elements or through the use of a shape sensing element in order to achieve a desired level of detail of detection so that, if desired, a care giver can determine both the position of the patient on the patient-carrying surface as well as the general outline of the patient's body on the patient-carrying surface in order to better monitor the patient.
According to the present invention, an apparatus is provided for detecting the presence or absence of a patient adjacent to a patient-carrying surface of a patient-support device. The apparatus includes a first sensing element spaced apart from the patient-carrying surface. The first sensing element defines a first detection space adjacent to the patient-carrying surface and provides a first input signal in response to the dielectric constant within the first detection space. The apparatus also includes a second sensing element spaced apart from the patient-carrying surface. The second sensing element defines a second detection space adjacent to the patient-carrying surface and provides a second input signal in response to the dielectric constant within the second detection space. A control unit provides an output signal indicating the presence or absence of the patient within the first detection space in response to the first input signal and within the second detection space in response to the second input signal.
The patient detection system can include one or more sensing elements. When a plurality of sensing elements are provided, the sensing elements can be spaced apart to define a plurality of separate, spaced-apart detection spaces and the care giver can monitor the position of the patient by monitoring the signals from each sensing element. It can be seen that by increasing the number of sensing elements in the patient detection system, the care giver can monitor the position of the patient with an increasing level of detail.
The patient detection system includes a plurality of sensors located adjacent the support surface. Each sensor having an output signal variable in response to the change detected by the sensor. A processor is provided having inputs of the output signals from the sensors. The processor monitors the output signals and provide an indication of change of position of the body relative to the support surface and to individually recalibrate each of the plurality of sensors. The automatic recalibration occurs upon sensing a substantial change due to an item being moved onto the surface. This change may be a body moving onto the surface. The processor may also sense changes in the support surface and recalibrate each of the sensors upon a detection of change in the support surface. This would accommodate for changes of pressure in a fluid support surface. The processor recalibrates all of the sensors for small changes existing for a pre-determined duration and for all large changes. The processor recalibrates all of the processors for a combination of changes in less than all of the sensors from the previous calibration for each sensor.
Wherein the support surface can be articulated, the system also includes a position sensor having an output couple the processor. The processor automatically recalibrates the system response to a change in the position of the articulated elements. The processor include an output coupled to a communication port for providing a nurse call system or other remote locations with an indication of change of the position of the body relative to the support surface.
A method according to the present invention of calibrating a plurality of sensors located adjacent the support surface comprises determining an initial calibration value for each sensor. Next, a determination of a change of signal of each sensor from its initial calibration is determined. Each of the plurality of sensors are individually recalibrated when a predetermined set of changes of the individual sensor signals has been determined. The predetermined set of changes may include, if a group of sensors, less than all of the sensors, each have a change above a first value. Also, the set may include if a group of sensors, less than all of the sensors, in total have changes above a second value.
While the patient detection system can provide information relative to the position of a patient on a sleeping surface of a bed, the patient detection system in accordance with the present invention can also be used with other patient-support devices such as stretcher, operating tables, chairs, and the like. For example, a chair including side-by-side first and second sensing elements beneath the seat cushion and side-by-side third and fourth sensing elements behind the back cushion can be used to provide the care giver with detailed information about the patient's posture and positioning on the chair.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best modes of carrying out the invention as presently perceived.